I would like to be able to use my stock list as an track library
It's great to have the Stock Management feature and also see how many tracks items remain in the chosen track library!
However in my stock I have parts from Roco and Fleischmann which means I have browse back and forth between track libraries when creating layouts. When editing layouts I see lots of track items I don't own which means I have to scroll around or browse track categories to find the track I want to add to my layout. Even with tracks from only one manufacturer I have to either switch between track classes (straight/curves/turnouts/..) or scroll a lot to find my tracks.
So, to summarise if I could select 'Stock' in the Track libraries menu I would only see what I have to work with and make editing faster. If I need a part not currently in my stock, I simply then browse to the full track library in question.

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Mats commented
I don't agree that this is duplicate of the issue linked above.
What is missing from the 'Custom Scale' capability is to support Stock lists with multiple manufactures. Now I only see parts from one manufacturer in the custom scale list at a time (it shows parts from the latest opened library).Steps:
1) I have 1 track from Fleischmann N (no roadbed) and one from TRIX N in my stock list.
2) I browse to Fleischmann N (no roadbed) in the Library Catalog
3) I select Custom Scale in the Library Catalog
4) Now I only see the Fleischmann track
5) Then I browse to Minitrix in the Library Catalog
6) I select Custom Scale in the Library Catalog
7) Now I only see the Trix track